This invention relates generally to a digital energy meter and, more particularly, to an improved digital electrical energy consumption meter which does not require the actual measurement of instantaneous current in order to obtain power consumption measurements of more than adequate accuracy.
The present invention pertains to the measurement of energy consumed by a load connected to an electrical transmission line that in turn is connected to an alternating current source or supply. Typical examples might be the supply of alternating current to a home or resident in the form of single phase alternating current, or to a commercial or industrial user in the form of a multiple phase alternating current supply.
The present invention further relates to the art of digital electrical power consumption meters which are desirable in general because an effective digital meter should normally provide a greater accuracy of measurement with less expensive manufacturing and the capability of easy remote reading of power consumption many miles from the meter by transmitting the digital information via radio waves or telephone lines or the like.
The voltage supply varies sinusoidally, approximately, with time. However the delivered current varies in time depending upon the nature of the load and the controls which regulate the load. For example, the current may be sinusoidal if the load consists of a heating element with some inductive characteristics. The current wave form will usually not match the voltage wave form instant by instant, but will be delayed slightly giving rise to a phase shift of current with respect to voltage. In addition nonsinusoidal current wave forms will result from certain load controls such as semiconductor controls, semiconductor light dimmers, for example.
Instantaneous power is the current multiplied by the voltage at any instant. The effective power is the integral of the instantaneous power over time, divided by the time over which the integration takes place. Digital type meters presently known deduce this from instantaneous power measurements. Energy consumption meters (watt hour meters) yield the integral of the power over time, or just the integral of instantaneous power over time, without the division mentioned above.
In power measurement procedures, prior art digital meters measure current and voltage separately, yet simultaneously, and multiply the current and voltage values to provide instantaneous power and then sum the results to give watt-hours. The values of voltage and current must each be digitized for this function. To achieve simultaneity the simple solution uses two digital to analog converters to make these two measurements of voltage and current thereby increasing the expense of manufacture.
Two difficulties of basic importance arise in implementation of known types of digital watt-hour meters. First, simultaneous readings of voltage and current are more costly than separately taken readings and it is obvious that cost must be kept to an absolute minimum when pertaining to meters that are going to be used in massive quantities. Second, the measurement of voltage is simple and very inexpensive, involving a simple resistor bridge or an equivalent, but current is measured on a line or conductor separated from a ground return line by a high voltage and direct readings of current are expensive.
The method and apparatus of the present invention eliminate these difficulties and provide a digital watt-hour meter of more than adequate accuracy which may be manufactured at acceptable cost.